cmd/compile: do not write slices/strings > 2g

cmd/compile: do not write slices/strings > 2g

The linker will refuse to work on objects larger than
2e9 bytes (see issue #9862 for why).

With this change, the compiler gives a useful error
message explaining this, instead of leaving it to the
linker to give a cryptic message later.

Fixes #1700.

Change-Id: I3933ce08ef846721ece7405bdba81dff644cb004
Reviewed-on: https://go-review.googlesource.com/74330
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>